Nervous System
Nervous System
The nervous system is divided into two major parts,
(1) the central nervous system (CNS) and
(2) the peripheral nervous system (PNS).
• Central Nervous System
– The central nervous system consists of the brain and the spinal
cord. Functions of the central nervous system include
integration, control, consciousness, and mental activity.
• Peripheral Nervous System
– The peripheral nervous system consists of all the nervous
system components, such as the nerves and neurons, that
extend from or are located outside of the central nervous system.
The peripheral nervous system is divided into the (1) sensory
(afferent) division and the (2) motor (efferent) division
– Afferent (sensory) division
• The sensory division involves information collected from the somatic
division, visceral division, and special senses and delivered to the
central nervous system (CNS).
– Efferent (motor) division
Anatomy and Physiology Text and Laboratory Workbook, Stephen G.
Davenport, Copyright 2006, All Rights Reserved, no part of this publication can
be used for any commercial purpose. Permission requests should be
addressed to Stephen G. Davenport, Link Publishing, P.O. Box 15562, San
Antonio, TX, 78212
• The efferent division involves information flow from the central
nervous system (CNS) to the somatic division and the visceral
division.
Nervous System
•
Somatic Division
– The somatic division of the peripheral nervous system is the division involved
with the voluntary control of body movements. The somatic division is divided
into the sensory (afferent) and the motor (efferent) components. The sensory
component functions in receiving stimuli and conducting information to the CNS
concerning voluntary body movements. The motor (efferent) component
functions to deliver information from the CNS to the skeletal muscles, thus,
directing their contraction.
•
Visceral Division
– The visceral division of the peripheral nervous system is the division involved
with involuntary control of body movements such as those of the cardiovascular,
digestive, urinary, respiratory, etc., systems. The sensory component functions in
receiving stimuli and conducting information concerning involuntary control to the
CNS. The motor component functions to deliver information from the CNS to
control involuntary movements. The motor component for involuntary control is
routed through the autonomic nervous system (ANS). The autonomic nervous
system, divided into the parasympathetic and sympathetic divisions, directs
motor control to smooth muscle, cardiac muscle, and glands.
•
Special Senses
– The special senses include taste, hearing, equilibrium, vision, and smell.
Information from the five senses is routed to the CNS by way of the afferent
(sensory) division. Information processed by the central nervous system may
produce an efferent (motor) response that is directed to either/both the efferent
somatic or visceral divisions.
Figure 17.1
An overview of the organization of the nervous system.
Neurons
• Neurons (nerve cells) are the cells of the
nervous system that function in
NEURONS
– (1) the generation and
– (2) the conduction of the nerve impulse, and the
– (3) secretion of a neurotransmitter at their terminals.
• Neurons have a cell body with one or more
processes (nerve fibers) extending from them.
1
Structure of a
Multipolar Neuron – Cell Body
• Neuron Cell Body
– The cell bodies of neurons are located in the gray
matter of the central nervous system and in structures
called ganglia in the peripheral nervous system. The
cell body of a neuron contains abundant cytoplasm
with numerous organelles. The organelles that are
easily observed with the light microscope include
• (1) a large nucleus that contains one to several dark-stained
nucleoli and
• (2) dark-stained granules called Nissl substance, or bodies
(rough endoplasmic reticulum).
Structure of a
Multipolar Neuron - Processes
• Dendrites
– Depending upon the type of neuron, one to many
processes called dendrites may be present at the cell
body.
– Dendrites have traditionally been described as the
neuron’s processes that function in the conduction of
impulses toward its body.
– Another description is that the dendrites are the
structures that function as the receptive portions of
the neuron. The latter function is the one used is this
study.
Figure 17.2
A “typical” multipolar neuron showing numerous processes
associated with the cell body. Only one axon originates at the
cell body, all the other processes are dendrites.
Structure of a
Multipolar Neuron - Processes
• Axon
– Usually, a neuron has only one process called an axon
associated with its cell body.
– The site on the cell body where the axon originates is called the
axon hillock.
– Once the axon leaves the cell body, the axon can split into
branches called collaterals.
– The axon ends in fine branches called telodendria. The end of
each tenodendrion is called an axon terminal that functions in a
synapse, the site of where the nerve impulse passes to another
neuron, a muscle, or a gland.
– Traditionally, an axon has been described as the portion of the
neuron that functions in the conduction of impulses away from
the cell’s body.
– Another description is that the axon is the process that
• generates and
• conducts the impulse, and
• releases a neurotransmitter at its axon terminals.
Classification According to
Function
CLASSIFICATION OF
NEURONS
Classification According to
Function
Three classifications of neurons according to function are
(1) sensory neurons, (2) motor neurons, and (3)
association neurons (interneurons).
• Sensory neurons
– Neurons that transmit impulses generated at their receptors
toward the central nervous system are sensory, or afferent,
neurons. They constitute the sensory (afferent) division of the
peripheral nervous system.
• Motor neurons
– Neurons that transmit impulses from the central nervous system
to effectors (glands and muscles) are motor, or efferent,
neurons. They constitute the motor (efferent) division of the
peripheral nervous system.
• Association neurons (Interneurons)
– Neurons of the CNS that transmit impulses from one neuron to
another are generally called association neurons, or
interneurons.
2
Classification According to
Structure
Three classifications of neurons according to structure are (1) unipolar
neurons, (2) bipolar neurons, and (3) multipolar neurons.
• Unipolar neurons
– A unipolar neuron has a single continuous fibrous process that is
associated with its body. Its single process is produced by the merging
of its receptive dendrites with the conductive axon that terminates with
synaptic contacts either in the brain or spinal cord. Unipolar neurons are
sensory neurons associated with the peripheral nervous system.
•
Bipolar neurons
– A bipolar neuron has two fibrous process, each process is associated
with the cell’s body. Bipolar neurons are sensory neurons associated
with the neural pathways of the senses involving sight, hearing, and
smell.
•
Figure 17.3
An overview of the functional classification of neurons.
Multipolar neurons
– A multipolar neuron has more than two processes associated with its
cell body. Only one of the processes is the axon, all of the other
processes are dendrites. Multipolar neurons are common in the CNS
functioning as association (interneurons), and exiting the CNS
functioning as motor neurons.
NEUROGLIA
Neuroglia are the cells and their
associated branching fibers that
support neural tissue
Figure 17.4
An overview of the structural classification of neurons.
Neuroglia
Neuroglia are the cells and their associated
branching fibers that support neural tissue.
• Central nervous system
– Four varieties of neuroglia found in the are
•
•
•
•
(1) ependymal cells,
(2) astrocytes,
(3) oligodendrocytes, and
(4) microglia.
• Peripheral Nervous System
– Two varieties of neuroglia found in the peripheral
nervous system are
• (1) satellite cells and
• (2) Schwann cells.
Neuroglia of the
Central Nervous System
• Ependymal cells
– Ependymal cells are found lining the cerebrospinal
fluid containing cavities of the CNS. In the brain
ependymal cells line the ventricles, and in the spinal
cord they line the central canal.
The ependymal
cells function in the production and regulation of
cerebrospinal fluid (CSF).
• Astrocytes
– Astrocytes, the most numerous glial cells, are glial
cells named for their star-shape. Among their
functions are support and nutrient
exchange/regulation between neurons and adjacent
capillaries.
3
Neuroglia of the
Central Nervous System
• Oligodendrocytes
– Oligodendrocytes are the glial cells that
produce the myelin sheaths of CNS axons.
The oligodendroglia produces sheet-like
extensions that form the myelin sheets.
• Microglia
– Microglia are phagocytic cells of the CNS.
They remove debris, waste, pathogens, and
other materials.
Neuroglia of the
Peripheral Nervous System
• Satellite cells
– Satellite cells are the glial cells that surround the cell
bodies of neurons in ganglia, the only sites in the
PNS that contain cell bodies of neurons.
• Schwann cells
– Schwann cells are the glial cells that are associated
with all axons in the PNS. Schwann cells either tightly
wrap axons to produce myelin sheaths (myelinated
axons) or remain in close association to produce
unmyelinated axons.
Lab Activity 1
Motor Nerve Cells
• Observe a microscopic preparation of
“Motor nerve cells, smear” (Nerve cells,
spinal cord smear).
– Identify the multipolar neurons and neuroglia
cells (nuclei).
– Locate an isolated neuron and identify its cell
body, nucleus, Nissl substance (bodies), and
processes.
Figure 17.5
Scanning power photograph of a multipolar neuron
from a slide preparation labeled “Motor nerve cells,
spinal cord smear.”
Lab Activity 1
Motor Nerve Cells
• Cell body
– The cell body contains most of the cell’s organelles and
cytoplasm. The cytoplasm contains the usual cell organelles
except centrioles (the lack of centrioles makes the cells
amitotic). The nucleus is easily observed with one to several
nucleoli. Dark-stained areas of rough endoplasmic reticulum
called Nissl substance (bodies) can usually be observed with
high magnification.
Lab Activity 1
Motor Nerve Cells
• Neuroglia
– The supporting cells of the nerve tissue, the
neuroglia, are seen as the dark-stained nuclei
distributed throughout the preparation. Mostly
consisting of astrocytes, the neuroglia are highly
traumatized during tissue preparation leaving their
nuclei scattered throughout the preparation.
• Cell processes
– Usually, on smear preparations the cells are severely
traumatized, which makes the microscopic identification of the
dendrites and the single axon difficult. The axon is long, may
show distal branches, and does not contain Nissl substance
(bodies). In comparison, the dendrites are short, have numerous
branches, and contain Nissl substance.
4
Figure 17.7
High power photograph of multipolar nerve cells
from a slide preparation labeled “Motor nerve cells,
spinal cord smear.”
Figure 17.6
Low power photograph of motor nerve cells from a slide
preparation labeled “Motor nerve cells, spinal cord smear.” The
general structure of multipolar neurons is observed.
Myelinated and Unmyelinated
Axons of the PNS
MYELINATED and
UNMYELINATED AXONS
Peripheral Nervous System
Figure 17.8
Illustration showing structural differences
between myelinated and unmyelinated
axons in the peripheral nervous system.
• In the peripheral nervous system, neuroglia called
Schwann cells are arranged sequentially along all
axons. The plasma membranes of Schwann cells
contain a phospholipid called myelin.
– If the Schwann cells surround and tightly wrap an axon, they
produce a myelinated axon with each Schwann cell producing an
area of concentrically wrapped plasma membrane called the
myelin sheath. The Schwann cell’s cytoplasm covered by the
plasma membrane is displaced outward to the myelin sheath
and forms the membranous covering of the fiber, the
neurolemma. Small gaps, called nodes of Ranvier, are formed
between adjacent Schwann cells.
– Unmyelinated axons are formed when Schwann cells do not
tightly wrap axons. In unmyelinated axons, a single Schwann
cell usually associates with several axons and only partially
encloses the axons.
Figure 17.9
Illustration showing structural differences between
myelinated and unmyelinated axons in the peripheral
nervous system.
5
Myelinated and Unmyelinated
Axons of the CNS
• In the central nervous system, the neuroglia
called oligodendrocytes associate with the
axons to form myelinated axons.
– Oligodendrocytes associate with several axons by
produce sheet-like extensions that wrap around, thus,
myelinating the axons.
– Mostly, myelinated fibers are organized into the areas
of white matter of the brain and spinal cord.
– Unmyelinated fibers are common in the gray matter of
the brain and spinal cord.
Figure 17.10
Illustration showing myelination of fibers (axons)
in the central nervous system by oligodendrocyte.
Lab Activity 2 –
Medullated Nerve
• Observe a microscopic preparation labeled
“Medullated nerve, teased.”
– A nerve is a part of the peripheral nervous system
that consists of parallel axons (fibers) and their
associated Schwann cells enclosed in connective
tissue wrappings.
– Teasing the nerve separates the axons (fibers) for
individual observation. Some preparations are
specifically prepared (treated with osmic acid) to
show the internal details of the myelin sheath, the
node of Ranvier, and the axon. Otherwise, the
preparation will usually show only the surfaces of the
Schwann cells and the nodes of Ranvier.
Figure 17.11
Illustration of teased nerve fibers.
Lab Activity 2 –
Medullated Nerve
• Schwann cells
– Schwann cells are located sequentially along the axon. Each
Schwann cell tightly wraps the axon to form a myelin sheath.
The Schwann cell’s cytoplasm surrounded by plasma membrane
are located to the outside of the myelin sheath and are called the
neurilemma.
Figure 17.12
High power photograph of teased nerve
fibers (at the node of Ranvier).
• Myelin sheath
– The myelin sheath is formed by the wrapping of the myelin
containing plasma membrane of the Schwann cell around the
axon. The myelin sheath is easily observed in preparations
treated with osmic acid. Myelin treated with osmic acid is darkly
stained. If not treated with osmic acid, myelin sheaths are
identified as “remnants.”
6
Lab Activity 2 –
Medullated Nerve
• Nodes of Ranvier
– Nodes of Ranvier are gaps formed between adjacent Schwann
cells. They allow exposure of the axon to the extracellular
environment.
NERVE
• Axon
– An axon is the process (branch) of a neuron which
• (1) generates and
• (2) conducts nerve impulses, and
• (3) releases neurotransmitter at its terminal synapses.
A nerve is a part of the peripheral nervous
system and consists of parallel axons (fibers)
and their associated Schwann cells enclosed
in connective tissue wrappings (sheaths).
– The portions of the axons in nerves are the long processes
(fibers) that function in conduction of nerve impulses.
Nerves
• Nerves may contain
Organization of a Nerve
•
•
Each axon (fiber) and its associated Schwann cells are surrounded
by a connective tissue sheath called the endoneurium. A connective
tissue sheath called the perineurium organizes individual fibers
(axons) and their associated endoneuria into groups called
fascicles. An outermost connective tissue sheath called the
epineurium organizes the fascicles into a nerve.
Epineurium
•
Perineurium
•
Fascicles
•
Endoneurium
– (1) only myelinated fibers,
– (2) only unmyelinated fibers, or
– (3) a combination of both.
• According to the directions of impulse conduction,
nerves are classified as
– (1) sensory - Sensory nerves contain fibers of sensory (afferent)
neurons that convey impulses to the central nervous system
– (2) motor - Motor nerves contain fibers of motor (efferent)
neurons and convey impulses away from the central nervous
system to effectors.
– (3) mixed (both sensory and motor) - Mixed nerves contain a
mixture of both sensory and motor fibers.
– The epineurium is the outer connective tissue sheath of the nerve. The
epineurium surrounds groups of fibers (axons) called fascicles.
– The perineurium is the connective tissue sheath that organizes fibers
into fascicles.
– Fascicles are groups of fibers surrounded by the connective tissue
sheath called the perineurium.
– The endoneurium is the inner connective tissue sheath that surrounds
each individual axon (fiber).
Organization of a Nerve
• Axons
– The axon functions in the generation and the conduction of
the nerve impulse, and releases a neurotransmitter at its
terminal synapses. The portions of the axons in nerves are the
long processes (fibers) that function in conduction of nerve
impulses.
– All axons of the peripheral nervous system are associated with
Schwann cells.
• In a section of a nerve, an axon is identified as a tiny dark circular
structure associated with a Schwann cell. Myelinated axons are
centrally located within the myelin sheaths of the Schwann cells.
Axons and their associated Schwann cells are surrounded by a
connective tissue sheath called the endoneurium.
Organization of a Nerve
• Schwann cells
– Schwann cells are associated with the axons of the peripheral
nervous system. Myelinated axons are formed by the wrapping
of the myelin containing plasma membranes of Schwann cells
around the axons. Unmyelinated axons are formed when a
Schwann cell associates with several axons and does not tightly
wrap the axons.
• Neurilemma
– The neurilemma is the membranous covering of a nerve fiber.
The neurilemma is formed by the thin region of cytoplasm and
the plasma membranes of the Schwann cells.
• Myelin sheath
– Myelin sheaths are the fatty sheaths that are formed by the tight
wrapping of the Schwann cells around the axon. Unless the
specimen is specifically stained for myelin (medullated nerve
with osmic acid), the myelin sheath is lightly stained and exists
as a “remnant.”
7
Figure 17.13
General structure of a nerve. A nerve is a part of the peripheral nervous
system and consists of parallel axons (fibers) and their associated Schwann
cells enclosed in connective tissue wrappings.
Figure 17.14
Illustration of a cross section of a nerve. A nerve is a part of the
peripheral nervous system and consists of parallel axons (fibers) and
their associated Schwann cells enclosed in connective tissue wrappings.
Lab Activity 3
Cross Section of a Nerve
• Observe a preparation of a “Nerve, c.s.” (c.s.- cross
section). A cross section of a nerve may be presented
singly on a slide preparation or may be accompanied by
a longitudinal section (l.s.). Preparations with both
sections are typically labeled “Nerve, c.s. & l.s.”
• Most general preparation of nerves are not prepared with
osmic acid, thus, do not show darkly stained myelin
sheaths. Instead the myelin sheaths are observed as
“remnants” and are mostly identified by location.
• Observe the preparation with scanning power and
identify the epineurium, perineurium, fascicles, axons,
and myelin sheaths.
Figure 17.16
Illustration of myelinated fibers from a cross section (c.s.) of a
nerve (high magnification). The myelin sheaths of the Schwann
cells are not well preserved and are identified as “remnants.”
Figure 17.15
Scanning power photograph of a cross section of a nerve.
Since general preparations are not treated with osmic acid, the
myelin sheaths are observed as “remnants.”
Figure 17.17
High power photograph of fibers (axons) from a cross section of a
nerve. Preparation does not show preserved myelin sheaths.
8
Lab Activity 4
Longitudinal Section (l.s.) of a Nerve
Figure 17.18
High power photograph of fibers (axons) from a cross section of
a nerve prepared with “Masson” stain. Preparation does not
show preserved myelin sheaths.
• Observe a microscopic preparation labeled
“Nerve, l.s.” A longitudinal section of a nerve is
most useful in showing the relationship between
the Schwann cells and the axon.
• As with the cross section, myelin sheaths are
observed only if the specimen was processed to
maintain the myelin (medullated nerve treated
with osmic acid). Otherwise, the myelin sheaths
exist as “remnants” and are lightly stained.
Figure 17.20
High power photograph of fibers (axons) from a
longitudinal section of a nerve.
Figure 17.19
Low power photograph of a longitudinal
section of a nerve.
Lab Activity 5
Cross Section of
Medullated (Myelinated) Nerve
• Observe a microscopic preparation labeled “Medullated
Nerve, c.s., osmic acid” (c.s.- cross section). The
observation of a nerve in cross section allows a study of
– (1) its connective tissue organization and
– (2) of axons.
• Myelinated axons are best observed in preparations of a
“medullated nerve” treated with osmic acid. Identify the
–
–
–
–
–
–
–
–
(1) epineurium,
(2) perineurium,
(3) endoneurium
(4) fascicles,
(5) axons,
(6) Schwann cells,
(7) neurilemma, and
(8) myelin sheaths.
Lab Activity 5 - Myelinated Nerve
(with myelin sheaths stained)
• Observe a preparation of a “Medullated Nerve,
osmic acid.” Treatment of medullated
(myelinated) nerve preparations with osmic acid
darkly stains the myelin sheaths.
• Observe the preparation with scanning power
and identify the
–
–
–
–
–
epineurium,
perineurium,
fascicles,
axons, and
myelin sheath.
9
Figure 17.21
Scanning power photograph of a cross section of a medullated nerve
prepared with osmic acid. Myelin sheaths are easy to identify because
osmic acid stains myelin black.
Figure 17.22
Illustration of myelinated fibers from a cross section of a medullated
nerve (high magnification). Schwann cells surrounding the axons have
regions called myelin sheaths and regions of cytoplasm with a
covering plasma membrane, the neurilemma.
SPECIALIZED NEURON
ENDINGS
Figure 17.23
High power photograph of medullated fibers (axons) from
a cross section of a medullated nerve. Most of the axons
are surrounded by thick myelin sheaths.
Receptors
• Specialized neuron endings of the peripheral
nervous system are found associated with the
(1) sensory (afferent) and (2) motor (efferent)
neurons.
• The sensory division of the PNS relies on
dendrites, the receptive portion of the axon, to
respond to stimuli.
– Dendrites may be modified into specialized structures
called receptors.
– The motor division of the PNS relies upon the
synaptic transmission of the nerve impulse from the
axon to the effector, a muscle or a gland.
Specialized neuron endings of the
peripheral nervous system are found
associated with the
(1) sensory (afferent) and
(2) motor (efferent) neurons.
Receptors
• Receptors are sensory endings which respond
to specific types of stimuli.
– A stimulus is a change that promotes a response. For
sensory receptors stimuli are mediated through a
change in the receptors environment.
– Among the stimuli that receptors respond to are
changes in temperature (thermoreceptors),
mechanical forces such as pressure and stretch
(mechanoreceptors), and chemicals such as acids
and electrolytes (chemoreceptors).
10
Effectors
• Effectors are the muscles and glands
controlled by the peripheral nervous
system.
– The axons of efferent neurons synapse with
effectors and rely upon a neurotransmitter to
mediate the flow of information (nerve
impulse).
– Synapses with muscles are the
neuromuscular junctions and with glands
the neuroglandular junctions.
Figure 17.24
Simplified neural pathway between a receptor (Pacinian
corpuscle) and an effector (neuromuscular junction - neuron
synapse with skeletal muscle fiber)
RECEPTOR
Pacinian Corpuscle
RECEPTOR
Pacinian Corpuscle
RECEPTOR
Pacinian Corpuscle
– Upon stimulation, the dendrite generates an electrical
potential called a graded potential.
– A graded potential is a local response (here restricted
to the dendrite), and produces an electrical signal that
has an intensity related to the strength of stimulation.
If the graded potential reaches an intensity sufficient
to stimulate the axon to threshold, an action
potential is produced and propagated to the axon’s
terminus, the axon terminals.
– The axon terminals in response to the action potential
release a neurotransmitter, which functions as a
chemical mediator for the transfer of the electrical
information.
• Among the many specialized receptors of the
peripheral nervous system, the Pacinian
corpuscle is large and distinct.
– Pacinian corpuscles are lamellated pressure
receptors (mechanoreceptor) mostly located deep in
the dermis of the skin and in the loose connective
tissues distributed throughout the body.
– A Pacinian corpuscle consists of a centrally located
dendrite (the receptive portion of the neuron)
surrounded by layers of flattened Schwann cells,
which are surrounded by a connective tissue capsule.
Lab Activity 6
Pacinian Corpuscle
• A Pacinian corpuscle has a dendrite
(receptive region) located in the center of
concentric layers of flattened Schwann
cells (lamellae), which is surrounded by a
connective tissue capsule.
11
Lab Activity 6
Pacinian Corpuscle
• Dendrite
– The centrally located dendrite is the receptive portion
of the Pacinian corpuscle.
• Capsule
– The capsule is the outer connective tissue layer of the
Pacinian corpuscle.
• Lamellae
– The lamellae are concentric layers of flattened
Schwann cells.
Figure 17.25
A Pacinian corpuscle is a pressure receptor (mechanoreceptor). It has
a dendrite (receptive region) located in the center of concentric layers
of flattened Schwann cells (lamellae), which is surrounded by a
connective tissue capsule.
Effector
Neuromuscular Junctions
•
EFFECTOR
•
Neuromuscular Junctions
•
The axon of a motor neuron may branch many times as it enters a
muscle. At the point where an axon approaches the muscle fiber
(cell), it branches into many small terminal branches (telodendria)
that end in knob-like axonal terminals. The site on the muscle cell
where the axon’s terminals come in close contact (synapse) with the
muscle fiber is called the neuromuscular junction.
A small space, called the synaptic cleft, separates the axon’s
terminals and the adjacent region of the muscle fiber, the motor end
plate. The motor end plate is the specialized region of the muscle
fiber’s plasma membrane that contains receptors for the
neurotransmitter.
Thus, the neuromuscular junction includes the axon’s
terminals, synaptic cleft, and the motor end plate.
Lab Activity 7
Neuromuscular Junction
• Observe a preparation labeled
“Neuromuscular junctions.” The
preparations of neuromuscular junctions
are of skeletal muscle fibers (cells).
Follow several axons, each to its junction
with a skeletal muscle fiber.
Figure 17.26
Scanning power photograph of neuromuscular junctions. A
neuromuscular junction consists of the axon’s terminals, a synaptic
cleft, and the motor end plate of the skeletal muscle cell (fiber)
12
Lab Activity 7
Neuromuscular Junction
• Axon
– The axons of motor neurons are the processes that
generate and conduct nerve impulses to the
neuromuscular junctions and at their axonal terminals
release neurotransmitter.
• Neuromuscular junction
Figure 17.27
High power photograph of a neuromuscular junction.
– The neuromuscular junction consists of the axon’s
terminals, a synaptic cleft, and the motor end plate of
the skeletal muscle cell (fiber).
Electrical Terminology
• Potential energy
– State of electrical energy as measured by the
potential to produce electrical effects
• Voltage (potential)
Physiology of Conduction
– Electrical measurement used to describe
electrical potential between two points.
• Current
– Flow of electrical charge and is due to the
electrical difference (voltage) between two points
• Resistance
– Opposition to electrical flow
• Insulators have high resistance
• Conductors have low resistance
Electrical Terminology
• How might the following
terms apply to these two
batteries?
– Potential energy
• Are both the same?
– Voltage
• Are both the same?
– Current
Electrical Terminology and the
Cell Membrane
• How might the following terms apply to
the illustrated cell membrane?
– Potential energy
– Voltage
– Current
– Resistance
Extracellular
• Do both produce the same?
Size AAA
Size D
– Resistance
• Does a battery contain a
“resister?”
Intracellular
13
Electrical Terminology and the
Cell Membrane
• How might the
following terms
apply to the
illustrated cell
membrane?
–
–
–
–
Extracellular
Potential energy
Voltage
Current
Resistance
• How might the
following terms
apply to the
illustrated cell
membrane?
–
–
–
–
Intracellular
Membrane Potentials
Extracellular
Electrical Terminology and the
Cell Membrane
Extracellular
Potential energy
Voltage
Current
Resistance
Intracellular
Resting Membrane Potential
• Ionic difference between
intracellular and extracellular
fluids
– Extracellular higher
concentration of Na+ (and Cl-)
– Intracellular higher
concentration of K+ and
negative proteins.
Net result is potential difference
between extracellular and
intracellular.
Intracellular
– Extracellular is positive (Na+)
– Intracellular is negative due to
negative proteins.
Membrane Potential Changes
Mechanical Channels
• If the resting membrane potential is to change
– must be a change in the distribution of positive
and/or negative charges; a redistribution of ions –
• Movement of ions can result when ions move
through channels which include
• Sodium channels (typical) open
when subjected to mechanical
stimulus
– Mechanically-gated (regulated) channels
• Open when subjected to a mechanical stimulus
– Voltage-gated (regulated) channels
• Open when subjected to an electrical stimulus
– Chemically-gated (regulated) channels
• Open when subjected to a specific chemical such as a
neurotransmitter or hormone
– Passive (leakage) channels
• Ions may leak through channels (or the phospholipid bilayer)
14
Channels
• Identify regions which are
– Mechanically gated
– Electrically gated
– Chemically gated
Generator Potential
• Local response (graded potential at stretch
receptor)
• Sodium ions move across membrane
• Interior becomes less negative (more positive)
• Depolarization (changes toward less negative
(positive) voltage
– May not reach threshold, thus no effect (action
potential)
– May reach threshold and produce an action
potential
Threshold and Action
Potentials
• Threshold
– Point of depolarization
(stimulation) which initiates an
effect (action potential)
– In this case the electrically-gated
Na+ channels open, (which are
adjacent to the active
mechanically-gated channels).
– The mechanically gated Na+
channels become inactive
• Action potential
– Not local; travels great distance
– Involves electrically-gated
channels
– Propagated along fiber (axon)
Depolarization as Na+ Moves Inward
• Receptor’s
Na+ channels
become
inactive
• Local current
opened
adjacent
electricallygated Na+
channels
(threshold)
• These channels
produce local
current
Adj
tN +
Generation of Action Potential
1. Resting membrane potential is
established
2. Depolarization phase
– Increase in sodium ion permeability
– Self propagating event
3. Repolarization phase
– Decrease in sodium ion permeability
– Increase in potassium ion permeability
•
•
Undershoot or after-hyperpolarization
occurs
Redistribution of sodium and potassium
by ATP driven sodium-potassium pump
Repolarization as K+ Moves
Out
• Local current
opens adjacent
electrically-gated
K+ channels
• K+ moves
outward and
repolarization
occurs
• Local currents
open adjacent
Na+ channels
• Action potential is
propagated to
adjacent forward
ti
15
Na+ / K+ Pump
• The Na+ / K+ reestablishes the
extracellular and
intracellular ionic
gradients
– Pump requires ATP
– Na+ is pumped outward
– K+ is pumped inward
Synapse
• Anatomical relationship between
neurons, or neurons and an effector
organ, and at which a nerve impulse is
transmitted through the action of a
neurotransmitter.
Synapse
Components of Synapse
• Consist of
– Presynaptic membrane of axonal terminal (synaptic
knob or bouton) which functions in the release of
neurotransmitter
– Postsynaptic membrane (of dendrite, postsynaptic
neuron, effector, organ, etc.) which houses receptors
for neurotransmitter
– Synaptic cleft of extracellular material between
presynaptic and postsynaptic membranes which
electrically isolates the membranes.
Termination of Neurotransmitter
• Enzymes associated with postsynaptic
membrane or present in cleft
• Reuptake by astrocytes into presynaptic
terminal where degraded by enzymes
• Neurotransmitter diffuses away from
synapse
2. Calcium ion
channels open
1. Action potential
arrives
3. Calcium ions promote
exocytosis of neurotransmitter,
calcium ions are quickly removed
4. Neurotransmitter binds
to postsynaptic receptors
5. Receptors allow passage
of specific type of ions
6. Depending upon ion movement
postsynaptic membrane is either
7. Neurotransmitter is deactivated depolarized (EPSP) or
hyperpolarized (IPSP)
by enzymatic action; some
components may be reused
IPSP and EPSP
• EPSP
• Excitatory postsynaptic potential results
when interior becomes more positive
• IPSP
• Inhibitory post-synaptic potential results
when interior becomes more negative
16
Synaps
e
Synaps
e
A
B
The result is “A” or “B”?
Which is produced?
A) action potential, B) IPSP, C) EPSP?
Summation
• Summation is the adding together of
synaptic potentials (SPs). Could be
EPSPs, IPSPs, or both EPSPs and
IPSPs.
• Temporal summation
– Pertaining to time; the quick succession of
SPs at a few synapses are summated
• Spatial summation
A
B
The result is “A” or “B”?
Which is produced?
A) action potential, B) IPSP, C) EPSP?
Mechanisms of Neurotransmitters
• Direct acting
– Channel linked receptors result in the opening
of ion channels –
– Alter membrane potential of target
– Can produce depolarization (sodium ions
move inward) and hyperpolarization
(potassium ions move outward)
– Pertaining to space; many SPs occur over
the postsynaptic membrane and are
summated
Mechanisms of
Neurotransmitters
• Indirect acting
– Involves G-protein complex
– Results in the production of a second
messenger
– Second messenger may influence enzymes to
Autonomic Regulation
• Activate or inactivate proteins (translation)
• Regulate gene activity (transcription)
• Regulate membrane ion channels and potentials
17
Autonomic Systems
Brain
Motor Division
(efferent PNS)
CNS
Parasympathetic
(autonomic, visceral)
Sympathetic
(autonomic, visceral)
Spinal cord
• Sympathetic
– “fight or flight response”
– Terminal neurotransmitter is epinephrine (E) or
norepinephrine (NE)
• Parasympathetic
Cranial nerves
Somatic
(skeletal muscle, voluntary)
PNS
Spinal nerves
Sympathetic
(sweat glands, involuntary)
– “resting and digesting,” or “rest and repose”
– Terminal neurotransmitter is acetylcholine (ACh)
• Organs
– May have dual innervations, response is excitation
by one system and inhibition by other system
– May have single innervations, response is promoted
or not promoted.
HUMAN BRAIN
• Four major regions of the human brain,
HUMAN BRAIN
Four major regions of the human brain,
(1) cerebrum,
(2) cerebellum,
(3) diencephalon, and
(4) brain stem.
Figure 17.28
Midsagittal view of human brain showing four major regions.
– (1) cerebrum,
– (2) cerebellum,
– (3) diencephalon, and
– (4) brain stem.
Figure 17.29
Lateral view of human brain.
18
Figure 17.30
Superior view of human brain.
Figure 17.32
Illustration showing a midsagittal section of the human brain.
Figure 17.31
Inferior view of human brain.
Figure 17.33
Photograph of a midsagittal section of the human brain.
Cerebrum
• The cerebrum is the largest part of the
brain.
CEREBRUM
The cerebrum is the largest part of
the brain.
– The cerebrum is divided into the right and left
cerebral hemispheres, which are connected
inferiorly by a large band of white matter, the
corpus callosum.
– The cerebrum functions include
• integrating somatic (body) sensory and motor
information,
• thought,
• memory,
• reason, and
• emotions.
19
Cerebrum
• Cerebral hemispheres
– The right and left cerebral hemispheres form the superior portion
of the brain. Externally, four lobes, the (1) frontal, (2) parietal, (3)
occipital, and (4) temporal, are named for both their associated
cranial bones and cerebral landmarks. Both hemispheres are
referred to as the cerebrum.
• Gyri
– Gyri are rounded elevated ridges on the surface of the cerebrum.
• Sulci
Figure 17.34
Functional areas (Brodmann areas) of the cerebrum.
– Sulci are shallow grooves on the surface of the cerebrum.
• Fissure
– Fissures are deep furrows. Two dominate fissures of the brain
are the longitudinal fissure and the transverse fissure.
Cerebrum
• Longitudinal fissure
– The longitudinal fissure is a deep groove that medially
divides the cerebrum into its right and left cerebral
hemispheres.
• Transverse fissure
– The transverse fissure separates the superiorly
located cerebrum from the inferiorly located
cerebellum.
• Central sulcus
– A central sulcus is the centrally located shallow
groove of each cerebral hemisphere that divides each
frontal lobe from each parietal lobe.
Cerebrum
• Frontal lobes
– A frontal lobe is the most anterior lobe of each
cerebral hemisphere. Each is separated posteriorly
from a parietal lobe by shallow groove, a central
sulcus.
– The functional regions of the frontal lobes include
• (1) somatic motor cortex that controls movement of skeletal
muscles,
• (2) a premotor cortex for learned (memorized) motor skills
and habits
• (3) a motor area (Broca’s area) for motor control of muscles
associated with speech,
• (4) cognition (process of knowing - awareness, perception,
reasoning, and judgment),
• (5) language centers for word association and meaning.
Cerebrum
Cerebrum
• Precentral gyrus
– The precentral gyrus of each frontal lobe is a rounded elevated
ridge located immediately anterior to each central sulcus.
• A precentral gyrus functions as the primary motor cortex and
houses the neurons (pyramidal cells) directly involved in conscious
control of skeletal muscles.
• Parietal lobes
– Each parietal lobe is located posterior to its associated central
sulcus and anterior to each occipital lobe.
– The primary function of the parietal lobes is housing the areas
that receive and integrate relayed somatic sensory
information, the somatosensory areas.
– The primary sensory (somatosensory) areas of the postcentral
gyri receive the somatosensory information.
– Posterior to each postcentral gyrus, the somatosensory
association areas function to integrate the sensory information
so that it is comprehensible.
– The parietal lobe also functions in sensory integration for spatial
visualization (visual attention) and manipulation of objects.
• Postcentral gyrus
– The postcentral gyrus of each parietal lobe is a rounded elevated
ridge located posterior to each central sulcus. A postcentral
gyrus functions as the primary sensory (somatosensory) cortex
as it houses the neurons that receive information relayed from
receptors in the skin and from a group of receptors distributed in
muscles, tendons, and joints, the proprioceptors.
• Occipital lobes
– The occipital lobes are located posterior to the parietal lobes.
Each occipital lobe is separated anteriorly from its associated
parietal lobe mostly by the parieto-occipital fissure. The function
of the occipital lobes is to house the visual cortex. The posterior
occipital lobe houses the primary visual cortex, the neurons that
receive information relayed from the visual fields of the eyes.
Anterior to the primary visual cortex, the visual association area
allows interpretation of received visual information.
20
Cerebrum
• Temporal lobes
Cerebrum
• Cerebral cortex
– Each temporal lobe is located laterally on each hemisphere.
Each is separated from the frontal and parietal lobes by the
lateral sulcus. The functions of the temporal lobes include the (1)
auditory areas, (2) language area (Wernicke’s area), (3)
memory, and the (4) ability to categorize objects.
– The primary auditory cortex is located in the superior portion of
each temporal lobe and receives information relayed from the
auditory receptors. Inferior to the primary auditory cortex the
auditory association area allows meaningful interpretation of the
auditory information.
– Wernicke’s area functions in recognition of spoken words.
• Lateral sulcus
– Each lateral sulcus separates each temporal lobe from its
associated frontal and parietal lobes.
– The cerebral cortex is the outer gray matter of the hemispheres.
It is composed mostly of neuron cell bodies and unmyelinated
fibers.
• Cerebral white matter
– The cerebral white matter is deep to the gray matter. It is
composed mostly of myelinated nerve fibers.
• Lateral ventricles
– The two lateral ventricles are large chambers, one of which is
located within each cerebral hemisphere. The lateral ventricles
communicate with the third ventricle, each by way of an
interventricular foramen. Each lateral ventricle houses the
choroid plexus, which extends from the third ventricle. The
choroid plexus is the site for the production and regulation of
cerebrospinal fluid (CSF). Cerebrospinal fluid fills the lateral
ventricles and drains into the third ventricle.
Cerebrum
• Corpus callosum
– The corpus callosum consists of fibers (white matter) that
connect corresponding areas of the right and left cerebral
hemispheres. It is located superior to the lateral ventricles and
deep in the longitudinal fissure. It functions in communication
between the right and left cerebral hemispheres.
• Fornix
– The fornix is an arched band of white matter located inferior to
the corpus callosum. It connects and functions in communication
between regions of the brain called the hippocampus (functions
in memory processes) and the hypothalamus.
• Septum pellucidum
– The septum pellucidum is a membrane that medially separates
the two lateral ventricles.
DIENCEPHALON
The diencephalon is a region of the brain
that is surrounded by the cerebral
hemispheres. It consists mostly of the
(1) thalamus,
(2) hypothalamus, and
(3) epithalamus.
Diencephalon
• Thalami
– The thalami are two interconnected regions of gray matter, the
right thalamus and the left thalamus, that form the superior
portions of the lateral walls of the third ventricle. A bridge of
fibers, the intermediate mass, passes across the third ventricle
and connects the two thalamic regions. The primary function of
the thalami is to relay incoming sensory information to various
regions of the cerebral cortex.
• Epithalamus
Figure 17.39
The diencephalon consists of the epithalamus, thalami,
and the hypothalamus.
– The epithalamus is located superior to the thalamus and forms
the thin roof of the third ventricle. The pineal gland (body)
extends outward from the posterior surface of the epithalamus.
• Pineal gland (body)
– The pineal gland is a posterior extension of the epithalamus. The
pineal gland functions as an endocrine gland releasing
melatonin, a hormone that functions in sleep cycles and
reproduction.
21
Diencephalon
• Hypothalamus
– The hypothalamus is located inferior to the thalamus.
It forms the inferior portions of the lateral walls and
the floor of the third ventricle. Among the functions of
the hypothalamus are:
• (1) regulation of body temperature,
• (2) sensations (drives) of hunger and thirst,
• (3) production of two hormones, antidiuretic hormone (ADH)
and oxytocin (OT), that are released at the posterior pituitary
gland,
• (4) production of regulatory hormones to control the anterior
pituitary gland,
• (5) and regulation of the autonomic nervous system
(especially involving the responses to stress and the
coordination of information flow between the pons and
medulla).
Diencephalon
• Infundibulum
– The infundibulum is a hollow stalk of tissue which extends from
the hypothalamus to the pituitary gland. The infundibulum serves
as a pathway for (1) fibers (axons) and (2) blood vessels that
leave the hypothalamus and enter the pituitary gland.
– Fibers (axons) from specialized endocrine producing neurons of
the hypothalamus pass through the infundibulum, terminate at
the posterior pituitary gland, and release the hormones (1)
antidiuretic hormone (ADH) and (2) oxytocin (OT).
– A specialized unit of blood vessels in the hypothalamus functions
to pickup regulatory hormones. The blood vessels pass through
the infundibulum and deliver the regulatory hormones to the
anterior pituitary. The regulatory hormones function in
controlling the secretory activity of the anterior pituitary.
Diencephalon
• Third ventricle
– The third ventricle is a narrow chamber centrally
located in the diencephalon. The third ventricle
houses a choroid plexus, a structure that produces
cerebrospinal fluid (CSF). In addition to functioning as
a site for the production of cerebrospinal fluid by its
choroid plexus, the third ventricle receives
cerebrospinal fluid from the lateral ventricles. From
the third ventricle cerebrospinal fluid drains into the
cerebral aqueduct of the midbrain.
BRAIN STEM
The brain stem is located between the
diencephalon and the spinal cord. Superiorly to
inferiorly, the brain stem consists of the
(1) midbrain,
(2) pons, and the
(3) medulla oblongata.
Brain Stem
• Midbrain
– Located immediately inferior to the diencephalon, the midbrain is
the most superior portion of the brain stem. Functions of the
midbrain include (1) serves as a pathway for ascending and
descending tracts (peduncles), (2) visual and auditory reflexes
(corpora quadrigemina), (3) control of muscle tone, (4) regulation
of cerebral nuclei, and (5) maintenance of consciousness. Two
cerebral peduncles (bundles of myelinated fibers) that contain
ascending and descending fiber tracts are located on the
midbrain's ventral-lateral surface. The posterior surface of the
midbrain is formed by four rounded elevations called the corpora
quadrigemina. The midbrain also houses several nuclei (red
nucleus and substantia nigra).
Figure 17.40
The brain stem consists of the midbrain, pons, and the medulla. The
corpora quadrigemina is the posterior region of the midbrain. The
cerebral aqueduct is a channel that delivers CSF to the fourth ventricle.
22
Brain Stem
• Corpora quadrigemina
Brain Stem
• Pons
– The corpora quadrigemina are four rounded
elevations that form the posterior surface of the
midbrain.
– The superior two elevations are the superior colliculi
and the inferior elevations are the inferior colliculi.
– The pons is the rounded bulge of the brain stem
located between the midbrain (superior) and the
medulla oblongata (inferior). The pons functions to
• (1) connect the cerebellum superiorly with the midbrain and
cerebrum and inferiorly with the medulla oblongata and
spinal cord,
• (2) contain nuclei for respiration and
• (3) contain nuclei of four cranial nerves (trigeminal- V,
abducens- VI, facial- VII, and vestibulocochlear nerves- VIII).
• The superior colliculi contain nuclei that mostly function in
visual reflexes, especially involving movement of the head,
neck, and eyes.
• The inferior colliculi contain nuclei that function in auditory
reflexes, especially involving the movement of the head,
neck, and extremities.
Brain Stem
•
Medulla oblongata
Brain Stem
•
– The medulla oblongata is located between the pons (superior) and the
spinal cord (inferior). Its ventral surface is composed of two ridges of
motor fibers called the pyramids. The medulla oblongata functions
include:
– (1) connects the spinal cord with the brain (medulla is connection into
the brain stem),
– (2) houses autonomic centers (nuclei) that function in the regulation of
respiration and circulation (cardiovascular center - control of heart rate
and force of contraction and blood vessel diameter (tone),
– (3) contains nuclei of five cranial nerves (vestibulocochlear-VIII,
glossopharyngeal- IX, vagus- X, accessory- XI, and hypoglossal- XII
nerves),
– (4) relay centers for somatic sensory information to the thalamus and
cerebellum, and
– (5) functions as the site where motor tracts (pyramidal tracts) from the
motor cortex cross to the opposite side, the decussation of the
pyramids.
Cerebral aqueduct
– The cerebral aqueduct is narrow channel within the midbrain that
connects the third and the fourth ventricle.
– The cerebral aqueduct drains cerebrospinal fluid from the third ventricle
to the fourth ventricle.
•
Fourth ventricle
– The fourth ventricle is located posterior to the pons. Superiorly, the
fourth ventricle is continuous with the cerebral aqueduct, and inferiorly it
is continuous with the central canal of the spinal cord.
– The fourth ventricle houses a choroid plexus, a structure that produces
cerebrospinal fluid (CSF).
– In addition to functioning as a site for the production of cerebrospinal
fluid by its choroid plexus, the fourth ventricle receives cerebrospinal
fluid from the third ventricle by way of the cerebral aqueduct.
– Cerebrospinal fluid moves out of the fourth ventricle into the
subarachnoid space, a space formed under the arachnoid meninx, a
membrane covering that surrounds the brain and the spinal cord.
Cerebellum
Cerebellum
The cerebellum is located
posterior to the pons and medulla and
inferior to the occipital lobes of the
cerebral hemispheres.
• The cerebellum is located posterior to the
pons and medulla and inferior to the
occipital lobes of the cerebral
hemispheres.
• The cerebellum functions include the
coordination of complex muscle
movements and the maintenance of
posture and balance.
23
Cerebellum
• Cerebellar hemispheres
– The cerebellar hemispheres are located one on each side of the
cerebellum’s central vermis.
• Vermis
– The cerebellar vermis is the central region of the cerebellum that
functions to connect the two cerebellar hemispheres.
• Folia
Figure 17.41
The cerebellum functions include the coordination of complex muscle
movement and the maintenance of posture and balance.
– Folia are the horizontally oriented rounded ridges of cerebellar
hemispheres. The folia are separated by narrow grooves called
sulci.
• Arbor vitae
– The arbor vitae are the branching areas of cerebellar white
matter.
VENTRICLES
The ventricles of the brain are
small interconnected internal cavities
that contain the choroid plexuses and
cerebrospinal fluid (CSF).
Figure 17.46
The ventricles of the brain.
Ventricles
• Lateral ventricles
– A large lateral ventricle is found within each cerebral
hemisphere. The two ventricles are medially
separated by a thin partition called the septum
pellucidum. The lateral ventricles contain a structure
that functions in the production and regulation of
cerebrospinal fluid (CSF), the choroid plexus. The
lateral ventricles (first and second) communicate with
the third ventricle each by an interventricular foramen,
which allows CSF to flow into the third ventricle.
• Third ventricle
– The third ventricle is a narrow chamber located within
the diencephalon. It receives CSF for its own choroid
plexus and from the lateral ventricles. From the third
ventricle CSF flows into the cerebral aqueduct to the
fourth ventricle.
Ventricles
• Fourth ventricle
– The fourth ventricle is located between the pons and
the cerebellum. It connects to the cerebral aqueduct
superiorly and is continuous with the central canal of
the spinal cord. The fourth ventricle receives CSF
from its own choroid plexus and from the cerebral
aqueduct. From the fourth ventricle CSF flows
through foramina into the subarachnoid space, where
the CSF then circulates around the brain and spinal
cord.
24
Meninges
• The meninges are the three membranes
that surround the brain (and spinal cord).
MENINGES
The meninges are the three
membranes that surround the
brain (and spinal cord).
– The meninges protect and isolate the brain
(and spinal cord) by their structure and the
spaces they form, and they provide routes for
blood vessels.
– From outer to inner the three meninges are
the
• (1) dura mater,
• (2) arachnoid, and
• (3) pia mater
Meninges
• Dura mater
– The dura mater is the outermost of the meninges. It is
a tough fibrous membrane that is continuous around
the brain and spinal cord. The outer surface of the
brains dura mater is attached to the periosteum (lining
of bone). The inner surface of the dura contains blood
vessels and specialized veins called dural sinuses.
– One of the dural sinuses, the superior sagittal sinus is
located at the mid-line of the dura’s superior surface.
In addition to receiving venous return, the superior
sagittal sinus receives cerebrospinal fluid (CSF). A
minute space, the subdural space, separates the dura
mater from the underlying arachnoid.
Meninges
• Arachnoid
– The arachnoid is the middle meninx. The
subarachnoid space contains cerebrospinal fluid and
the arachnoid trabeculae. The arachnoid trabeculae
are a network of collagen and elastic fibers that
function to support the arachnoid and the maintain the
subarachnoid space. Cerebrospinal fluid functions to
protect the spinal cord and allow diffusion of various
chemical substances.
• Pia mater
– The pia mater is the innermost of the meninges. It is a
delicate vascular membrane that adheres to the brain
(and spinal cord).
Cerebrospinal Fluid
• Cerebrospinal fluid, or CSF, is produced by the
choroid plexuses located in the brains ventricles.
CEREBROSPINAL FLUID
Cerebrospinal fluid, or CSF, is
produced by the choroid plexuses
located in the brains ventricles.
– A choroid plexus is a vascular proliferation that
produces CSF by a combination of vascular filtration
and regulation by neuroglia called ependymal cells.
– CSF exits the fourth ventricle by way of three
foramina and enters the subarachnoid space, where it
circulates around the brain and spinal cord.
– Specialized areas of the arachnoid meninx, the
arachnoid granulations, penetrate the dura mater of
the superior sagittal sinus and allow drainage of the
CSF into venous circulation.
25
Cranial Nerves
• There are twelve pairs of cranial nerves,
each named and numbered with a roman
numeral.
– Except for the first two cranial nerves, ten
pairs emerge from the brain stem.
Figure 17.47
Overview of circulation of cerebrospinal fluid.
Sheep Brain Dissection
DISSECTION OF THE
SHEEP BRAIN
Because of its small size, ease of storage,
and most importantly, its anatomical
simularites to the human brain, the preserved
sheep brain is ideal for dissection
Sheep Brain Dissection
EXTERNAL ANATOMY
• Because the sheep is a quadruped, directional
terms of reference are obviously different for the
sheep brain than for the human (biped). Liberty
is taken and direction terms that apply to the
sheep brain in biped orientation are used (for
example, anterior (biped) is substituted for
cranial (quadruped).
EXTERNAL VIEWS
Sheep brains may be purchased with all the meninges
and the pituitary gland present or absent. It is
important at this time to make observations to
determine which meninges are present and whether
or not the pituitary is present.
26
Meninges
• The brain and spinal cord are surrounded
by three connective tissue membranes
called the meninges.
• From outer to inner, they are the
Dura mater
• Observe the brain for the presence of the dura
mater, the outer meningeal membrane which
covers the brain as a tough opaque membrane.
– If the membrane is present, you will not be able to
directly observe the surface of the brain. (If the dura
mater is not present on the preserved brain, proceed
to the identification of the arachnoid).
– Use scissors and carefully make an incision across
the dorsal surface of the brain about an inch back
from the brain’s anterior margin.
– Along the midline of the brain, observe that the dura
forms a large sagittal sinus (blood vessel) and forms
a partition between the two cerebral hemispheres.
– (1) dura mater,
– (2) the arachnoid, and
– (3) the pia mater. Sheep brains may be
purchased with or without the dura mater (and
arachnoid). All brains have the pia mater.
Arachnoid
• All brains should have an intact arachnoid.
The arachnoid appears as a thin semitransparent layer on the surface of the
brain.
– Blood vessels in the sulci are observed under
the arachnoid.
Figure 17.49
Superior surface of the sheep brain
showing the dura mater.
Pia mater
•
Figure 17.51
Anterior surface of the sheep brain showing the arachnoid.
All brains should have an intact pia
mater. The pia mater is a delicate
membrane which adheres to the surface of
the brain. Blood vessels may be seen
associated with it. The pia mater may be
separated from the brain by carefully
picking the surface of the brain with a
probe.
27
Figure 17.53
Lateral surface of the sheep brain.
Figure 17.52
Superior (dorsal) surface of the sheep brain.
External Structures
• Gyri
External Structures
• Longitudinal fissure
– Gyri are elevated ridges on the surfaces of the cerebral
hemispheres.
• Sulci
– The longitudinal fissure is a deep groove that medially divides
the cerebrum into its right and left cerebral hemispheres.
• Cerebellum
– Sulci are shallow grooves on the surfaces of the cerebral
hemispheres.
• Cerebrum (cerebral hemispheres)
– The right and left cerebral hemispheres form the dorsal portion of
the brain. Both hemispheres are referred to as the cerebrum
and are separated by the longitudinal fissure. Without tearing
the two hemispheres apart, carefully separate the fissure and
observe the corpus callosum. The corpus callosum connects the
two cerebral hemispheres.
– The cerebellum is located posterior to the cerebrum. It is
separated from the cerebrum by the transverse fissure.
• Transverse fissure
– The transverse fissure separates the cerebrum from the
cerebellum.
• Spinal cord
– The spinal cord is not part of the brain. It is seen extending
posteriorly from the brain stem.
External Structures
• Pituitary gland
– The sheep brain is purchased either with or without the pituitary
gland. The pituitary gland is observed as a small, dome-shaped
mass of tissue protruding from the ventral surface of the brain. It
is located immediately posterior to the optic chiasma. (If the brain
does not have a pituitary gland, proceed to the next
identification).
– If the brain was purchased without the complete dura mater, a
small remnant of the dura is preserved around the pituitary
gland.
– Without removing the pituitary, gently lift its posterior aspect
(along with the dura mater) away from the brain. Notice that it is
attached anteriorly to the brain (hypothalamus) by a small stalk,
the infundibulum. Remove the pituitary and observe the hollow
infundibulum. Remove any remaining dura mater.
Posterior View
•
Without tearing the cerebrum and the
cerebellum apart, carefully bend the
cerebellum away from the cerebrum to
expose the
– pineal gland and the
– corpora quadrigemina.
28
Figure 17.54
Posterior surface of the sheep brain.
Figure 17.55
Photograph of posterior surface of the sheep brain.
External Structures
• Corpora quadrigemina
– The corpora quadrigemina are four rounded
elevations on the posterior surface of the midbrain.
• Pineal gland (body)
– The pineal gland (body) is a rounded gland (body)
that extends from the posterior border of the
epithalamus.
Figure 17.56
Photograph of inferior surface of the sheep brain with dura
mater surrounding the pituitary gland.
Ventral Views
• Observe the inferior aspect of the sheep
brain. Observe the brain to determine if
the brain has the pituitary gland attached
to its inferior surface (read previous
discussion of “pituitary gland”).
Figure 17.57
Photograph of inferior surface of the sheep brain
with dura mater removed from the pituitary gland.
29
External Structures
• Longitudinal fissure
– The longitudinal fissure is a deep grove that medially divides the
cerebrum into its right and left cerebral hemispheres.
• Olfactory bulb
– The olfactory bulbs are located on the inferior surface of the
frontal lobes of the cerebrum. They receive the olfactory nerve
fibers which pass through the cribriform plate of the ethmoid
bone from the olfactory epithelium.
• Optic nerve
– The optic nerves are sensory nerves whose axons arise from the
retina.
• Optic chiasma
– The optic chiasma is the area where the two optic nerves meet.
Axons from the medial aspect of each eye cross over to the
opposite side.
Figure 17.58
Photograph of inferior surface of the sheep
brain the pituitary gland removed.
External Structures
• Optic tract
– The optic tract is the pathway of axons into the brain from the
optic nerve and the optic chiasma.
• Mamillary body
– The mamillary body is a rounded body that protrudes from the
ventral surface of the hypothalamus.
• Infundibulum
– A stalk of the hypothalamus that is located posterior to the optic
chiasma and anterior to the mamillary body. It provides a
connection between the hypothalamus and the posterior lobe of
the pituitary gland (the pituitary gland is not shown on the
illustration).
External Structures
• Brain Stem
– The brain stem is located between the diencephalon
(epithalamus, thalamus, and hypothalamus) and the spinal cord.
Anteriorly to posteriorly, it consists of the midbrain, pons, and the
medulla oblongata.
• Midbrain
– The midbrain is located posterior and slightly inferior to the
diencephalon, which is centrally located between the two
cerebral hemispheres. The ventral surface is composed of
ascending and descending fiber tracts called cerebral peduncles.
• Pons
– The rounded bulge of the brain stem is located between the
midbrain (anterior) and the medulla oblongata (posterior).
External Structures
• Medulla oblongata
– The medulla oblongata is located between the pons (anterior)
and the spinal cord (posterior).
• Spinal cord
– The spinal cord is not part of the brain. It is seen extending
posteriorly from the brain stem.
• Cranial nerves
– There are twelve pairs of cranial nerves. Most of the nerves are
small and delicate, and many were torn away from the brain
during its removal. Carefully inspect the brain and identify as
many cranial nerves as possible.
INTERNAL ANATOMY
The internal anatomy of the sheep brain is
studied from a brain cut in equal right and
left halves, a
(1) midsagittal section, and a
(2) transverse section.
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Midsagittal Section of
Sheep Brain
Figure 17.60
Illustration showing structures of midsagittal section.
Figure 17.59
A midsagittal section of the sheep brain showing four major regions of
organization: (1) cerebrum, (2) cerebellum, (3), diencephalon, and (4) brain stem
CEREBRUM
and Associated Structures
Figure 17.61
Photograph of dissected sheep brain showing structures of midsagittal section.
Cerebrum and Associated Structures
• Cerebrum (cerebral hemispheres)
– The right and left cerebral hemispheres form the
dorsal portion of the brain. Both hemispheres are
referred to as the cerebrum.
• Gyri
– Gyri are rounded elevated ridges (convolutions) on
the surface of the cerebrum.
• Sulci
– Sulci are shallow grooves (furrows) on the surface of
the cerebrum.
The cerebrum is the largest part of the
brain. The cerebrum is divided into the right
and left cerebral hemispheres, which are
connected inferiorly by a large band of white
matter, the corpus callosum.
Cerebrum and Associated Structures
• Transverse fissure
– The transverse fissure is a deep furrow that separates the
cerebrum and the cerebellum.
• Cerebral and cerebellar cortex
– The cortex is the outer gray matter of the cerebral hemispheres
and the cerebellum. It is composed mostly of neuron cell bodies
and unmyelinated fibers. Observe the outer gray matter of the
cerebellum, the cerebellar cortex.
• Cerebral and cerebellar white matter
– The white matter is deep to the gray matter. It is composed
mostly of myelinated nerve fibers. Observe the inner white
matter of the cerebellum, the arbor vitae.
• Lateral ventricles
– The two lateral ventricles are large chambers, one of which is
located within each cerebral hemisphere. They are filled with
cerebrospinal fluid.
31
Cerebrum and Associated Structures
• Corpus callosum
– The corpus callosum consists of fibers (white matter) that
connect corresponding areas of the right and left cerebral
hemispheres. It is located superior to the lateral ventricles and
deep in the longitudinal fissure.
DIENCEPHALON
and Associated Structures
• Fornix
– The fornix is an arched band of white matter located inferior to
the corpus callosum. It connects a region of the brain called the
hippocampus with the hypothalamus.
• Septum pellucidum
– The septum pellucidum is a membrane that medially separates
the two lateral ventricles.
The diencephalon mostly
consists of the (1) epithalamus (2)
thalami (pleural of thalamus), and
hypothalamus.
Diencephalon
• Thalami
– The thalami consists of two interconnected regions of gray
matter, the right thalamus and the left thalamus, that form the
superior portions of the lateral walls of the third ventricle. A
bridge of fibers, the intermediate mass, passes across the third
ventricle and connects the two thalamic regions.
• Epithalamus
– The epithalamus is located superior to the thalamus and forms
the thin roof of the third ventricle. The pineal gland (body)
extends outward from its posterior surface.
• Hypothalamus
– The hypothalamus is located inferior to the thalamus. It forms
the inferior portions of the lateral walls and the floor of the third
ventricle.
BRAIN STEM
and Associated Structures
The brain stem is located between the diencephalon and the
spinal cord. Anteriorly to posteriorly, it consists of the
(1) midbrain,
(2) pons, and the
(3) medulla oblongata.
• Third ventricle
– The third ventricle is a narrow chamber centrally located in the
diencephalon.
Brain Stem
•
•
The brain stem is located between the diencephalon and the
spinal cord. Anteriorly to posteriorly, it consists of the (1) midbrain,
(2) pons, and the (3) medulla oblongata.
Midbrain
– The midbrain is located immediately posterior to the diencephalon. On
its anterior surface are the two cerebral peduncles which contain
ascending and descending fiber tracts. On its posterior surface are four
rounded elevations called the corpora quadrigemina.
•
Corpora quadrigemina
•
Pons
– The corpora quadrigemina are four rounded elevations on the posterior
surface of the midbrain.
– The pons is the rounded bulge of the brain stem located between the
midbrain (anterior) and the medulla oblongata (posterior).
•
Medulla oblongata
– The medulla oblongata is located between the pons (anterior) and the
spinal cord (posterior).
Brain Stem
• Cerebral aqueduct
– The cerebral aqueduct is a narrow channel that
connects the third and the fourth ventricles.
• Pineal gland (body)
– The pineal gland (body) is a rounded gland that
extends from the posterior border of the epithalamus.
• Fourth ventricle
– The fourth ventricle is located inferior to the
cerebellum. It connects to the cerebral aqueduct
superiorly and is continuous with the central canal of
the spinal cord.
32
Cerebellum
• Cerebellum
CEREBELLUM
– The cerebellum is located superior to the pons and
medulla and posterior to the cerebral hemispheres.
• Arbor vitae
– The arbor vitae are the branching areas of cerebellar
white matter.
Transverse Section of Sheep
Brain
Figure 17.62
Transverse section of the sheep brain.
Transverse Section of Sheep Brain
• Cerebral cortex
– The cerebral cortex is the outer gray matter of the hemispheres.
It is composed mostly of neuron cell bodies and unmyelinated
fibers. Observe the cortex with a human brain model that has
been sectioned to show the interior of the hemispheres.
• Cerebral white matter
– The cerebral white matter is deep to the gray matter. It is
composed mostly of myelinated nerve fibers. Observe the white
matter with a human brain model that has been sectioned to
show the interior of the hemispheres.
• Choroid plexus
– A choroid plexus is the site of cerebrospinal fluid production. A
choroid plexus is located in third ventricle and extends into the
two lateral ventricles. Additionally, a choroid plexus is found in
the fourth ventricle. A choroid plexus consists of capillaries and
glial cells called ependymal cells.
SPINAL CORD
The spinal cord functions to receive, integrate,
and transmit information to and from the
peripheral nervous system, upper and lower
levels of the cord itself, and the brain.
33
Spinal Cord
• The spinal cord is housed within the vertebral
canal of the vertebral column.
– The spinal cord begins at the base of the brain stem
with the termination of the medulla oblongata.
– The spinal cord continues through the vertebral canal
to its termination at the tip of its conus medullaris.
– In the adult, the cone shaped ending of the spinal
cord, the conus medullaris, is located at the level of
the first-second lumbar vertebrae.
– At the tip of the conus medullaris a fibrous strand, the
filum terminale extends to the second sacral
vertebra and functions to anchor the spinal cord.
Meninges of Spinal Cord
• Surrounding the spinal cord are three
membranes called the meninges. The
meninges protect and isolate the spinal
cord (and brain) by their structure and the
spaces they form. From outer to inner the
three meninges are the
– (1) dura mater,
– (2) arachnoid, and
– (3) pia mater.
Spinal Cord
– Along the spinal cord, structures called spinal roots
merge to form the spinal nerves.
– There are 31 pairs of spinal nerves with each pair
(except spinal nerves designated C1, which exit
between the skull and the first cervical vertebra)
exiting the vertebral column between adjacent
vertebrae. Thus, the spinal nerves are named by their
vertebral associations forming the spinal nerves C1 C8, T1 - T12, L1- L5, and S1 - S5. (In the adult the
regions of the spinal cord and associated spinal
nerves don’t anatomically align with the regions of the
vertebral column because during youth, the spinal
column grows faster than the spinal cord.)
Meninges of Spinal Cord
• Dura mater
– The dura mater is the outermost of the meninges. It is
a tough fibrous membrane that is continuous around
the spinal cord and brain. In the spinal cord the dura
is separated from the surrounding vertebrae by the
epidural space.
– The epidural space mostly contains adipose tissue
and serves as a passageway for blood vessels. The
dura mater surrounding the brain is attached to the
periosteum (lines cranial bones) and thus, does not
present an epidural space. A minute space, the
subdural space separates the dura mater from the
underlying arachnoid.
Meninges of Spinal Cord
• Arachnoid
– The arachnoid is the middle delicate meninx. The
subarachnoid space contains cerebrospinal fluid
(CSF) and the arachnoid trabeculae. The arachnoid
trabeculae are a network of collagen and elastic fibers
that function to support the arachnoid and the
maintain the subarachnoid space. Cerebrospinal fluid
functions to protect the spinal cord and allow diffusion
of various chemical substances.
• Pia mater
– The pia mater is the innermost of the meninges. It is a
delicate vascular membrane that adheres to the
spinal cord (and brain).
Figure 17.64
Illustration showing the relationship of the meninges
and spinal cord to a vertebra (cervical region).
34
Spinal Cord
• The inside of the cord is organized into gray
matter and its outside into white matter.
• The gray matter is divided into
–
–
–
–
Figure 17.65
Scanning power photograph showing the relationship
of the meninges and spinal cord in a vertebra.
(1) the posterior horns,
(2) the lateral horns,
(3) the anterior horns, and
(4) the gray commissure.
• The white matter is divided into funiculi
(columns), which are named according to their
position as the
– (1) posterior,
– (2) lateral, and
– (3) anterior funiculi.
Spinal Cord
• Spinal roots are located along the cord.
Figure 17.66
Illustration showing the general structure of the spinal cord.
– The dorsal root is located dorsally and consists of
axons of sensory neurons. The dorsal root is
associated with an enlarged region called the dorsal
root ganglion. The dorsal root ganglion houses the
cell bodies of sensory (unipolar) neurons.
– The ventral root is located ventrally and consists of
axons of motor neurons.
– The dorsal and ventral roots unite to form the spinal
nerves. A pair of spinal nerves exit the vertebral
column between adjacent vertebrae (except spinal
nerve, C1).
– The spinal nerves are components of the peripheral
nervous system
Lab Activity 13
Spinal Cord
Figure 17.67
Illustration showing the relationship of sensory and motor
neurons to the organization of the spinal cord.
• Observe a microscopic preparation labeled
“Spinal cord, cross section.” Different quality
microscopic preparations are available. Some
are special “ultra thin” sections with special
stains for advanced study, and others are for a
general anatomical study.
• Also, there are a variety of preparations from
different locations along the cord. They range
from having no spinal roots, to parts of roots, to
complete roots with spinal nerves.
35
Figure 17.68
Illustration of the general structure of the spinal cord as
seen in cross section through the spinal ganglia.
Structures of Spinal Cord
• Posterior median sulcus
– The posterior median sulcus ia a posterior, medial, groove that
partially divides the cord into right and left sides. It is narrower
than the anterior median fissure. Often on histology slides it
appears as a single line because it was closed by adherence of
the sides.
• Anterior median fissure
– The anterior median fissure is an anterior, medial furrow that
partially divides the cord into right and left sides. It is wider than
the posterior median fissure.
Figure 17.69
Scanning power photograph of a cross section of a spinal cord
through the spinal ganglia (“Spinal cord ganglion, silver”).
Structures of Spinal Cord
• Dorsal roots
– The dorsal roots contain axons of sensory neurons that synapse
with the cell bodies of association neurons located in the
posterior (dorsal) horns. The dorsal roots may not be present on
your preparation.
• Dorsal root ganglia
– Each dorsal root has a dorsal root ganglion that consists mostly
of cell bodies of sensory neurons and neuroglia. The cell bodies
of these sensory neurons are unipolar and are surrounded by
neuroglia cells. If a dorsal root ganglion is present, observe it
under high magnification. It consists of the cell bodies of
sensory (unipolar) neurons and their associated neuroglia cells
(satellite cells). The axon directly associated with the cell body
may have been cut away.
• Ventral root
– The ventral roots are the pathways for the axons that leave
motor neurons located in the anterior (ventral) horns.
Figure 17.70
Photograph showing the relationships of the dorsal root, dorsal root
ganglion, ventral root, and spinal nerve to the spinal cord.
Figure 17.71
Photograph showing the detail of the dorsal root ganglion. Inset
shows the cell body of a unipolar neuron (sensory neuron).
36
Spinal Cord – Gray Matter
• The gray matter is located to the inside of the
cord and consists mostly of multipolar neurons
(mostly association and motor neurons),
unmyelinated fibers, and neuroglia.
Figure 17.72
Scanning power photograph of the junction between
the gray matter and the white matter.
– The gray matter is often described as being in the
shape of “butterfly wings” or the letter “H.”
– The two posterior projections are called the posterior
(dorsal) horns, and the two anterior projections are
called the anterior (ventral) horns.
– Lateral horns are seen in the thoracic and upper
lumbar regions.
Spinal Cord – Gray Matter
• Anterior (ventral) horns
– The anterior projections of gray matter are called the anterior
(ventral) horns. They contain mostly the cell bodies of motor
neurons and neuroglia.
• Posterior (dorsal) horns
– The posterior projections of gray matter are called the posterior
(dorsal) horns. They contain mostly association neurons
(interneurons) and neuroglia.
• Gray commissure
– The gray commissure is the gray matter that connects the right
and left gray masses (horns) together. It encloses the central
canal.
Figure 17.73
High power photograph of the gray matter of the spinal cord.
Spinal Cord – White matter
• The white matter is located to the outside of the
spinal cord’s gray matter.
– The white matter contains mostly myelinated axons
(some unmyelinated axons) and neuroglia. Some
pathways are established by horizontal axons from
one side of the cord to the other.
– However, most pathways are formed by axons that
are arranged along the longitudinal (vertical) axis of
the cord. These axons establish pathways of
ascending and descending tracts.
Figure 17.74
High power photograph of the white matter of the spinal cord.
Most of the axons are cut in cross section and show
“remnants” of their myelin sheaths.
37
Spinal Cord – White matter
• The tracts of the white matter are organized into three
major white columns, or funiculi: the posterior, lateral,
and anterior funiculi.
• Posterior funiculi
– The posterior funiculi (dorsal white columns) are located at the
posterior aspect of the white matter and contain ascending
tracts.
• Lateral funiculi
– The lateral funiculi (lateral white columns) are located at the
lateral aspects of the white matter and contain ascending and
descending tracts.
• Anterior funiculi
– The anterior funiculi (anterior white columns) are located at the
anterior aspect of the white matter and contain ascending and
descending tracts.
38